When a reverse voltage is abruptly applied to a diode in normal operation, a reverse current flows for a while. This is because the reverse voltage causes the carriers stored in the diode to be discharged outside due to the conductivity modulation of the carriers. This reverse current is known as the recovery current of a diode. The peak value (I.sub.rp) of the recovery current increases as the time variation (di/dt) of the current becomes abrupt in a change from the forward to the reverse direction.
The diode breaks down when di/dt increases excessively at the time of recovery. Generally, the value of di/dt at which the breakdown occurs (hereinafter called the "critical di/dt") is required to be large. In the case of a diode with a low critical di/dt, it has heretofore been the practice to limit the di/dt in the diode by inserting an inductance in the main circuit or by holding down the switching speed of the switching elements used together with the diode in order to protect the diode from breakdown at the time of recovery.
When such an inductance is inserted in the main circuit, however, the voltage generated at the time of recovery of the diode, or the switching of the switching elements, becomes excessively high, and thus the diode and the switching element breakdown; therefore, a protective circuit such as a snubber circuit is needed. Nevertheless, there arise the problems of making a power converting apparatus large-sized, of lowering the conversion efficiency and so forth, because the provision of a protective circuit results in not only increasing the number of parts, but also the incurring of a power loss.
The turn-on time of the switching element needs to be extended to decrease the di/dt in the diode by slowing down the switching speed of the switching element, which would make it necessary to limit the switching frequency and incur an additional cost for cooling the elements because the turn-on loss of the elements increases.